The invention concerns a nuclear spin resonance (NMR) measuring device, in particular an NMR tomography apparatus with a preferentially superconducting main field coil which, in a measurement volume whose center coincides with the origin of a Cartesian x, y, z coordinate system, can generate a homogeneous static magnetic field B.sub.0 in the direction of the z-axis of the coordinate system and with a tesseral gradient coil system for the production of magnetic gradient fields with a largely linear dependence in the measuring volume in a direction perpendicular to the z-axis, whereby the gradient coil system is comprised of at least four largely identical saddle-type partial coils which are arranged symmetrically with radial and axial separation from the origin of the coordinate system, each coil exhibiting two electrically conducting segments which run in the azimuthal direction about the z-axis of which one segment has as small a radial separation r.sub.1 and the other as large a radial separation r.sub.2 as possible from the z-axis.
Such an NMR measuring device is known for example from the publication DE 40 29 477 A1.
Whereas tomography systems in the past have been nearly exclusively used for diagnosis, in the future there will be an ever-growing need for combined systems with which therapeutic measures can be immediately followed and checked by tomography apparatuses. A multitude of therapeutic measures, for example, surgery in particular invasive microsurgery or irradiation thereby require as free an access to the patient as possible. This is, however, hindered in conventional NMR-systems by all three field producing components, namely the main magnetic field magnet, the gradient coil system as well as the RF-resonator.
With respect to the main field magnet the problem has already been solved with the magnet system known from DE 39 07 927 A1 exhibiting Helmholtz coil-like transverse field coil which, due to its construction, limits free sideward access to the measuring volume only in a particularly minor fashion.
In order to facilitate the carrying out of a minimum invasive method (so-called "key-hole-surgery") by which, due to the lack of a direct field of view onto the operation region in consequence of the, under certain circumstances, minuscule operating opening in the patient, an NMR monitoring provides the operator with important assistance for the on-line observation of the operation, the transverse access, with as large a sideward access angle as possible, to the measuring volume inside the NMR-apparatus should not finally be limited by the gradient coil system.
Known for example from the publication EP-A 0 073 402 is a gradient coil system in the form of simple or composite saddle coils which lie pairwise across each other on an azimuthal section about the z-axis. One such gradient coil system is located in the axial bore of the main field magnet and usually penetrates the axial region about the coordinate origin which also includes a sideward space (gap) kept open by the special construction, mentioned above, of the main field magnet and the RF-resonator. Since the known transverse gradient coils, in particular the shielding coils which are normally used in conjunction therewith, have their largest winding density precisely in the region of the central plane z=0, the advantage of transparency and the possibility of manipulations on the patient offered by the special form of the main field magnet and the RF coil system are completely lost. On the other hand the spatial arrangement of the known saddle coils which are arranged on a cylindrical surface about the z-axis under the boundary condition that no portion of the coil projects into the gap region, would lead to unacceptably large non-linearities of the produced gradient fields, to very weak gradient strengths and to large stray fields in the region of the cryostat, i.e. to the production of eddy currents when switching the gradients which, for their part, act to the detriment of the homogeneity of the static magnetic field B.sub.0 in the investigational volume.
Known from the above-mentioned publication DE 40 29 477 A1 are tesseral gradient coils for NMR tomography apparatuses with which partial coils of the gradient coil system lie across from each other symmetric to the z=0 plane and to a plane perpendicular thereto, by way of example y=0, the coils each exhibiting two azimuthal segments with differing radial separations r.sub.1 and r.sub.2 from the z-axis each of which having the same z-position. Through this configuration the parasitic magnetic field, generated by the gradient coils, with field components perpendicular to the z-axis which induces currents in the object under investigation and in the cryostat of the main field magnet is significantly reduced. With a set of two coil types of this kind, at both sides (relative to the zy-plane) of the above-mentioned NMR measuring device investigational volume access gap, it is theoretically possible to construct a gradient coil system with the desired properties of sufficient linearity of the produced gradient fields, a relatively small stray field, and an unconstrained transverse access to the measuring volume. In order to produce the usual gradient strengths (typically at least 10 m tesla/m at a current of circa 250 A) such a coil system must exhibit approximately 30 windings with a conductor cross section of approximately 60 mm.sup.2. These windings would be preferentially arranged, with respect to the z-axis, axially behind each other, whereby a sizeable axial extension of such a coil of approximately 180 mm would occur. In order to achieve the desired produced gradient field linearities, an azimuthal extent of the segments about the z-axis of at least circa 120.degree. is necessary so that the z- and y-gradient coils would penetrate into each other, i.e. a configuration of a system with gradient coils for both spatial directions would not be realizable. On the other hand, instead of an axial distribution, a positioning of the windings radially within another would, for its part, lead to a dramatic deterioration of the linearity of the produced gradient fields as well as to a strong reduction in the gradient strength per unit current.
It is therefore the underlying purpose of the invention to improve an NMR measuring device with a gradient coil system of the above-mentioned kind in such a fashion that x- and y-gradients can be simultaneously produced and that the tesseral gradient coils on the one hand produce as linear a magnetic gradient field as possible in the measuring volume, on the other hand, however, only marginally limit or do not limit whatsoever a sideward or diagonal access to the measuring volume and thereby open up a maximum free access to the measuring volume.